1. Field of the Invention
This invention pertains to apparatus for measuring respiration volume and to devices utilized in the calibration of such apparatus.
2. Statement of the Prior Art
Non-invasive apparatus for measuring respiration volume are known. See, for example, the apparatus disclosed in commonly owned application Ser. No. 102,408, filed Dec. 11, 1978, now U.S. Pat. No, 4,308,872 entitled Method and Apparatus for Monitoring Respiration, now abandoned, the contents of which is incorporated herein by reference in its entirety. In use, such apparatus generally requires calibration for each patient prior to use. That is, the measurements recorded on the apparatus must be calibrated against one or more initial measurements based on a known volume of expired air. Spirometers are generally used for this purpose.
While the construction details of spirometers vary widely, they generally comprise a closed chamber having inlet and outlet openings hermetically separated by a plate or the like disposed for sliding movement in the chamber. As the patient breathes into the inlet port, the plate is forced towards the outlet port. The volume of air expired by the patient is proportional to the movement of the plate. Thus, respiration volume can be measured by suitable indicia on the chamber. Alternatively, plate movement can be detected by a transducer, which then provides a signal proportional to respiration volume. The signal may be recorded, displayed as a numerical read out, or both. An exemplary spirometer is disclosed in U.S. Pat. No. 3,848,583 issued to Parr.
While spirometers provide reasonably accurate respiration volume measurements, they are unduly complex and expensive for the limited purpose of calibrating non-invasive respiration volume measuring apparatus. Indeed, their main application is for detecting respiration volume directly.
U.S. Pat. No. 3,821,950 issued to Boehringer and U.S. Pat. No. 3,810,461 issued to McCormick each discloses a flexible, air-tight bag for measuring respiration volume. The bag is provided with a mouthpiece through which the patient breathes into the bag. Thereafter, the bag is disposed on a flat surface, such as a table, and means such as a cylindrical tube is utilized to roll the bag from one end to the other such that the air trapped in the bag is forced into one end. The rolling is continued until it is apparent that the uncoiled portion of the bag is substantially filled with air. The volume of air in the bag is determined by suitable indicia on the bag.
The principal drawback of these devices is that there is a substantial possibility of "ball-valving" upon inhalation. That is, upon inhalation from the bag, the mouthpiece may become occluded by the bag as the bag collapses. As a result, air may become entrapped in the bag. Inasmuch as it is essential to accurate respiration volume measurements that the bag be completely empty before expiration, such entrapment is undesirable. Furthermore, considering that the devices disclosed in these patents are intended as disposable monitors of respiration volume, they are unduly complex insofar as they require means for preventing the escape of air through the mouthpiece after expiration is completed so that the bag can be placed on a flat surface for rolling.
U.S. Pat. No. 3,512,521 issued to Jones discloses a breath isolator disposable between the patient and a spirometer for preventing cross contamination of patients as a result of multiple use of the spirometer. The device comprises a closed chamber having an inlet opening at one end in which a mouthpiece is fitted. Inside the chamber, the mouthpiece extends into a flexible bag. The end of the chamber opposite the mouthpiece is provided with an outlet opening which communicates, via a flexible hose, with the spirometer. It will therefore be apparent that upon expiration into the mouthpiece, the bag inflates, thereby forcing an equal volume of air from the chamber through the outlet opening and into the spirometer. While the device disclosed in the patent apparently serves its intended purpose, it is unnecessarily complicated and, therefore, uneconomical as well.